Display devices utilizing liquid crystal light modulation with varying colors

ABSTRACT

A nematic liquid crystal device which is electrically controlled in order to produce colors which can be displayed for entertainment or advertising purposes. The liquid crystal material is sandwiched between a pair of parallel transparent plates coated with transparent conducting films. The coated plates are rubbed in the same direction such that the nematic molecules align parallel to the glass surface and point in the rubbed direction. The liquid crystal sandwich structure is then placed between linear polarizers which are oriented either parallel or crossed with respect to each other and at 45* with respect to the rubbed direction. By applying an electric field across the liquid crystal material via the aforesaid transparent conducting films, the color observed through the polarizers can be made to vary, depending upon the magnitude of the electric field. Furthermore, by applying a continually varying voltage across the conducting films, the color can be made to continually vary also.

.. vv DH United States l ll Fl 7 Harsch DISPLAY DEVICES UTILIZING LIQUIDCRYSTAL LIGHT MODULATION WITH VARYING COLORS [75} Inventor: Thomas B.Hal-sch, Stow, Ohio [73] Assignee: International Liquid Xtal Company,

Cleveland, Ohio [22] Filed: July 15, 1971 [21] Appl. No.: 162,833

[52] US. Cl. 350/150, 350/160 LC, l78/5.4 BD

1 Jan. 15, 1974 Primary ExaminerEdward S. Bauer AttorneyBrown, Murray,Flick & Peckham [57] ABSTRACT A nematic liquid crystal device which iselectrically controlled in order to produce colors which can bedisplayed for entertainment or advertising purposes. The liquid crystalmaterial is sandwiched between a pair of parallel transparent platescoated with transparent conducting films. The coated plates are rubbedin the same direction such that the nematic molecules [51] IIII. Cl.602T l/16 alig parallel to the glass surface and point in the [58] Fleldof Search 350/150, 160 LC rubbed direction, The liquid crystal sandwichstructure is then placed between linear polarizers which are [56]References C ted oriented either parallel or crossed with respect toeach UNITED STATES PATENTS other and at 45 with respect to the rubbeddirection. 3,694,053 9/1972 Kahn 350 150 By m g an electric across theliquid crysfal 355L026 12/1970 H il i 350/150 material via the aforesaidtransparent conducting 3,544,659 3/ I951 Dreyer .I 350/148 films, thecolor observed through the polarizers can be 3,627,408 l2/197l Fergason350/160 X made to vary, depending upon the magnitude of the 3.625,59l12/1971 Freiser et al---. 35 0 electric field. Furthermore, by applyinga continually 3,656,834 4/1972 Haller et al. 350/15 varying voltageacross the conducting films, the C0101 can be made to continually varyalso.

9 Claims, 7 Drawing Figures was; 7-0 I p.- w

DISPLAY DEVICES UTILIZING LIQUID CRYSTAL LIGHT MODULATION WITH VARYINGCOLORS BACKGROUND OF THE INVENTION As is known, there are a large numberof organic chemical compounds that will, within a particular temperaturerange, exhibit nematic-phase liquid crystals. These compounds are liquidin the sense that their molecules are not dissociated as in a gas nor sotightly bound within a structure as to constitute a solid. At the sametime, they are said to be crystalline, in that there is a particularordering to the orientation of the molecules, as is sometimes evidencedby peculiar optical effects.

It is also known that when a nematic-phase liquid crystal material issandwiched between transparent plates that have been rubbed, each ofthem unidirectionally and on the surface in contact with thenematicphase liquid crystal material, there is obtained a liquid crystalunit whose optic axis lies in the direction of unidirectional rubbing.For example, if the rubbed directions are parallel to each other, thealignment that is produced by the rubbing is such that the long axis ofthe nematic molecules align parallel to the glass surface and point inthe rubbed direction. If the rubbed directions are placed at angles withrespect to each other, the result will be an optical media which rotatesthe plane of polarization by an amount equal to the angle between therubbed directions. When an electric field is applied across the liquidcrystal material thus sandwiched between rubbed plates, the naturalalignment of the molecules effected by the rubbing can be varied,depending upon the magnitude of the electric field.

SUMMARY OF THE INVENTION In accordance with the present invention, anematicliquid crystal device is provided for producing varying colorswherein the nematic-liquid crystal is sandwiched between two sheets oftransparent conductive glass each of which is rubbed in the samedirection such that the liquid crystal material orients uniformly whenplaced between the conductive glass plates. The alignment that isproduced by the rubbing is such that the long axis of the nematicmolecules align parallel to the glass surface and point in the rubbeddirection. The nematic-liquid crystal that is used must have a maximumdielectric constant lying along the long molecular axis. That is, theliquid crystal material must have a positive directric anisotropy.

When an electric field of increasing intensity is applied across the twoplates of conductive glass, the molecules will be distorted as the fieldincreases from their parallel position to a position where they arenearly normal to the glass plates, the instantaneous angle between themolecular axes and the final normal position depending upon themagnitude of the electric field. If the liquid crystal cell is placedbetween crossed linear polarizers with the rubbed direction at 45 to thepreferred axes of the polarizers, with no applied field it will appearnearly white in color when viewed through one of the polarizers becauseof the high birefringence of the cell when the molecules are parallel tothe glass surfaces. When an electric field of sufficient magnitude tocause the molecular axes to align normal to the glass plates is applied,no light will be transmitted. However, if the magnitude of the electricfield is then decreased continuously from that upper value, the color oflight transmitted will be first blue, then purple, green, yellow,orange, red, white and then repeat itself as the field is decreasedfurther. When the magnitude of the electric field becomes too small, thebirefringence of the sample becomes too large and the transmissioncolors wash out.

Hence, the electric field strength must be kept between certain criticalvalues to produce pleasing colors. By choosing an electric fieldstrength of sufficient predetermined magnitude, any color can bedisplayed. Furthermore, if a varying alternating current voltage isapplied across the liquid crystal cell, the instantaneous color willchange in relationship to the magnitude of the electric field appliedacross the cell. Therefore, it is possible in accordance with theinvention to produce a time varying color with a time varying electricfield. The colors can also be produced using parallel polarizersdisposed at an angle of 45 with respect to the rubbed direction exceptthat with a given field strength, the color obtained will becomplementary to that obtained with cross polarizers.

The above and other objects and features of the invention will becomeapparent from the following detailed description taken in connectionwith the accompanying drawings which form a part of this specification,and in which:

FIG. 1 is a schematic cross-sectional view of a liquid crystal unit madein accordance with the present invention;

FIG. 2 is a view illustrating the manner in which the transparent platesof the liquid crystal unit of FIG. I are rubbed in the same direction;

FIG. 3 is a schematic illustration showing the manner in which polarizedlight passes through the liquid crystal unit of the invention;

FIG. 4 illustrates one manner in which the display device of theinvention may be used with a ground glass or plastic screen;

FIG. 5 is an illustration of another manner in which the invention maybe used to project varying colors;

FIG. 6 is a schematic circuit diagram showing one manner in which apotential can be varied across the transparent conducting plates onopposite sides of a layer of liquid crystal material by the use of amicrophone or other similar device; and

FIG. 7 is a schematic illustration of the manner in which the liquidcrystal unit of the invention may be utilized in accordance with theinvention to convert a black and white television picture into a colorpicture.

With reference now to the drawings, and particularly to FIG. 1, there isshown a liquid crystal unit 10 comprising a first transparent plate 12,preferably of .glass, and a second transparent plate 14, also of glassand extending parallel to the plate 12. The plates 12 and 14 are spacedapart by suitable spacers, not shown, by approximately 0.001 inch,although the spacing may be varied to suit requirements. This space isfilled with a layer 16 of nematic-phase liquid crystal material with apositive dielectric anisotropy. Preferably, the liquid crystal material16 is orie comprising major portions such as 20 to percent each ofbis-4-noctyloxybenzal)-2-chlorophenylenediamine andpmethyIbenzaI-p'-n'butylaniline, these making up about 60 to 97 percentof the total composition and pcyanobenzal-p'-n-butylaniline comprisingthe remaining 3 to 40 percent. The liquid crystal material is moreparticularly described in copending application Ser. No. 113,948, filedFeb. 9, 1971.

Disposed on the interior surfaces of the transparent plates 12 and 14and in contact with the liquid crystal material 16 are coatings l8 and20 of thin transparent electroconductive material, such as the known tinoxide or indium oxide coatings. These coatings are quite thin and highlyresistive, for example, on the order of 150 ohms per unit square orabove, and possibly as high as 5,000 to 10,000 ohms per unit square. Itis desirable that the transparent electroconductive coating be of thekind that is applied at relatively low temperatures, such as about 500F, by the process of cathode-sputtering in a vacuum, so that dangers ofwarpage may be safely avoided.

In FIG. 2, there is shown a view of the plates 12 and 14 which maycomprise flat glass on the order of about 1/8 inch thick having thelayers 18 and 20 of the transparent conducting material deposited on thefacing surfaces thereof. In the preparation of a liquid crystal unit inaccordance with the invention, the layers of transparent conductingmaterial that are in contact with the nematic-phase liquid crystalmaterial 16 must be prepared by being stroked or rubbedunidirectionally, with, for example, a cotton cloth. The direction ofrubbing on the respective plates 12 and 14 is indicated by the lines 22and 24 in FIG. 2; and it will be appreciated that the directions ofrubbing on the respective plates are parallel to each other. The effectof this, as explained above, is to align the liquid crystal moleculessuch that the long axes of the molecules are parallel to the glasssurface and point in the rubbed direction.

Now, if an electric field is established across the liquid crystal layer16 by applying a potential between the conducting plates 18 and 20, themolecules tend to align with the electric field. Since the liquidcrystal has a positive dielectric anisotropy, the molecules can bedistorted from their parallel position to an extreme position where theyare nearly normal to the glass plates, the angle between the molecularaxes of the molecules and the lines 22 and 24 depending upon themagnitude of the electric field.

If the liquid crystal cell is placed between crossed linear polarizers26 and 28 as shown in FIG. 3, with the rubbed direction (i.e., thedirection of lines 22 and 24) being at 45 to the preferred axes of thepolarizers, then there will be maximum transmission of light through thepolarizers 26 and 28. When no electric field is established across theliquid crystal layer 16, it will appear nearly white in color because ofthe high birefringence of the cell. At this time, the molecules areparallel to the glass surfaces and aligned with lines 22 and 24.However, when an electric field is applied of sufficient magnitude todistort the molecular axes of the molecules, light from a polychromaticsource 30, when viewed after passing through the polarizers and cell 10,will change color. Furthermore, as the intensity of the electric fieldis increased by means of a variable voltage source 32 (also shown inFIG. 1) the colors viewed will change until the electric field reaches apoint where the molecular axes of the liquid crystal molecules arenormal to the glass plates. At this point, no light is trans mitted.

If the electric field is now decreased in magnitude via the variablevoltage source 32, the color of light transmitted decreases from a verydark blue to a very light blue, followed by magenta, yellow, green, andred. A further decrease in the field then results in the sequence ofmagenta, yellow, green and blue repeating itself in progressivelynarrower color bands until a lower minimum field is reached where themagnitude of the electric field becomes too small and the birefringenceof the sample becomes too large such that the transmission colors washout.

Referring again to FIG. 1, the polarizers 26 and 28 can be in the formof flat sheets, preferably dichroic polarizing sheets of the typemanufactured by Polaroid Corporation. However, other types of polarizersmay be used to suit requirements. For that matter, instead of usingseparate polarizing sheets or separate polarizers, the polarizers can bedirectly incorporated into the cell 10. In this regard, the surfaces ofthe conductive coatings l8 and 20, for example, can be rubbed andtreated with a solution of a dye which forms a dichroic film asdescribed in Dreyer U. S. Pat. Nos. 2,544,659; 2,524,286 and 2,400,877.Such a solution can comprise a 4% aqueous solution of methylene blue. Bycoating the rubbed surface of the conductive coatings 18 or 20 with thisdye solution and allowing it to dry, a dichroic film will be depositedon the surface with a thickness on the order of about 1 micron. Byplacing the liquid crystal material as described above between the tworubbed plates treated with polarizing material, a single layer materialwill result which will have the complete system incorporated therein.

As a specific example, and assuming that the width of the liquid crystallayer 16 is about 1.5 mils, the voltage from source 32 applied acrossthe cell can be increased to about volts where no light is transmitted.Then, as the voltage is decreased and the electric field likewisedecreased between the conducting films l8 and 20, a dark blue colorexists which gradually becomes lighter as the voltage is decreased toabout 50 volts. Upon further reduction in the voltage, and when thevoltage reaches about 37.5 volts, the color changes to magenta and thenyellow, followed by green at about 32.5 volts. At 29 volts blue againoccurs, followed by magneta at 27 volts; whereupon the sequence ofmagenta, yellow, green and blue repeats itself with the color bandsbeing closer and closer to each other until about 5 volts is appliedacross the liquid crystal cell, whereupon the colors wash out.

From the foregoing, it can be seen that a selected color can be viewedthrough the assembly of FIG. 1 or 3, for example, by applying aparticular voltage across the liquid crystal cell. Furthermore, byapplying an alternating current voltage across the transparentconducting layers 18 and 20, the colors can be made to change as the RMSvalue of the alternating current voltage varies. This, of course, willproduce a continual change in color in what can be compared to apsychedelic effect. Preferably, the alternating current voltage appliedacross the cell, in the case where the liquid crystal layer is about 1.5mils in thickness, is in the range between about 25 and 50 volts wherethe widest color bands occur.

One manner in which the liquid crystal cell of the invention can be usedto produce an advertising display is shown in FIG. 4. Thus, the cell 10is interposed between a polychromatic source of light 34 and a groundglass or plastic screen 36 which is viewed by the eye 38 of an observer.As the voltage from variable voltage source 32 is varied, the colorsviewed on the screen 36 will also vary, producing a more or lesspsychedelic effect, depending upon the rate of change of the voltage.Furthermore, it will be appreciated that by changing the voltage appliedto the liquid crystal cell in steps, different, fixed colors can be madeto appear on the ground glass screen 36 which can be over-printed withadvertising material or the like.

The invention can also be used as a filter for colored illumination orspotlighting, such as illustrated in FIG. 6 wherein light from apolychromatic light source 38 is focused into a beam which passesthrough the liquid crystal cell 10, similar to cell 10 shown in FIG. 1.In this case, the output of the variable voltage source 32 will bevaried in steps. Since the colors appear at discrete values of voltage,the color obtained can be calibrated with the applied voltage. That is,the color can be selected by setting the voltage across the cell at agiven value.

Another method of producing pleasing colors and color patterns is to useconvergent light for observation. If with the physical configurationsdescribed previously, highly convergent light is used for illumination,the interference pattern characteristic of a uniaxial material isobserved. Since the direction of the optic axis is continuallychangingwith the electric field variation, the interference figure changes withtime. In white light the interference figure is colored and gives veryinteresting patterns. The convergence of the light may either beproduced before or after the light passes through the liquid crystalcell. The light may be made convergent before passing through the cellby imposing a convergent lens system after the light source.Interference patterns may also be produced by placing a diverging lenssystem in front of the cell, thus forming a virtual image of theinterference figure.

The variable voltage source 32 can be replaced by means whereby thecolor of the light can be made to vary in accordance with the amplitudeof music, the voice, or some other transducer which produces analternating current output. Thus, as shown in FIG. 6, a microphone 42responsive to sound, such as music or talking, is connected through anaudio amplifier 44 to a variable gain amplifier 46. The output of thevariable gain amplifier is applied through a high voltage driver 48 tothe liquid crystal cell 10. The output of amplifier 46 is also appliedto a peak detector 50 which, in turn, controls the gain of amplifier 46.In this manner, it will be appreciated that as the amplitude of theaudio signal varies, so also will the color displayed by the liquidcrystal cell 10. If desired or necessary, the output of the amplifier 44can be applied to a rectifier and the rectified audio signal applied tothe liquid crystal cell. The peak detector 50 serves to regulate thegain of amplifier 46 such that the signal falls within the desired rangeas betwen about 25 and 50 volts in the example given above where thethickness of the liquid crystal layer is about 1.5 mils.

In FIG. 7, still another use of the invention is shown for converting ablack and white television picture into a colored picture. This isapplicable, for example, in color television wherein three successiveframes of a television picture are scanned on red, green and bluephosphors, respectively. Thus, a black and white television receiver 52is provided with output leads 54, 56 and 58 on which the synchronizingpulses for the red, green and blue frames appear in sequence. Thesesynchronizing pulses, while appearing on the received television signal,will not affect the receiver 52 since it is a black and white receiverhaving only a single electron gun and a black and white phosphor on theface of its receiving tube. The signals on leads 54, 56 and 58 areapplied through separate amplifiers and clippers 60, 62 and 64,respectively, to three gate circuits 66, 68 and 70. Also applied to thegate circuits 66, from a tap on voltage divider 72, for example, is avoltage which, when applied to a liquid crystal cell 67 similar to cell10 shown in FIG. 1, will transmit the color blue. Similarly, the gate 68is connected to a tap on voltage divider 72 such that the voltageapplied to the gate 68 is that required to produce green with the liquidcrystal cell 67; while the voltage applied to gate 70 from the voltagedivider 72 is that necessary to establish an electric field across thecell 67 which will produce the color red. The voltage divider 72 issupplied by the voltage source 74, as shown.

In the operation of the system of FIG. 7, the gates 66-70 are turned ONin sequence by the synchronizing pulses at the output of circuits 60-64.As each gate is turned ON, the liquid crystal cell 66 will transmit acolor determined by the voltage derived from the voltage divider 72.Thus, the colors red, green and blue will occur in sequence veryrapidly, faster than the eye can follow the change, whereby a compositecolor picture will be produced from the three basic colors red, greenand blue. The intensity of the color at any point on the picture will,of course, depend upon the intensity of the light produced at that pointby the black and white receiver 52.

Although the invention has been shown in connection with certainspecific embodiments, it will be readily apparent to those skilled inthe art that various changes in form and arrangement of parts may bemade to suit requirements without departing from the spirit and scope ofthe invention.

I claim as my invention:

1. In combination, a layer of nematic liquid crystal material ofpositive dielectric anisotrophy disposed between transparent parallelplates, said plates being coated on the surfaces thereof which face eachother with films of transparent conducting material, said films oftransparent conducting material being rubbed unidirectionally to providerubbed lines with the lines on one film being parallel to those on theother whereby the long molecular axes of the liquid crysal moleculeswill normally align parallel to the surfaces of the plates and point inthe direction of said rubbed lines, polarizers on opposite sides of saidlayer of liquid crystal material extending essentially parallel to saidplates to provide a sandwich structure through which light can pass, andmeans for providing a variable electric field between said transparentconducting films whereby the color of light passing through saidsandwich structure will vary depending upon the magnitude of said field.

2. The combination of claim 1 wherein said liquid crystal materialcomprises a mixture of bis-(4-noctyloxybenzal-Z-chlorophenylenediarnineand pmethylbenzal-p ri'butylaniline, these making up about to 97 percentof the total composition and pcyanobenzal-p-n-butylaniline comprisingthe remaining 3 to 40 percent.

3. The combination of claim I wherein said polarizers comprisepolarizing sheets on the sides of said plates opposite said liquidcrystal material.

7. The combination of claim 1 wherein said polariz ers are crossed at anangle of 45 with respect to said rubbed lines on the transparentconducting films.

8. The combination of claim 1 wherein said polarizers have their majoraxes both extending parallel to the rubbed lines on said transparentconducting films.

9. The combination of claim 1 wherein said electric field is varied insteps.

4 k i i

2. The combination of claim 1 wherein said liquid crystal materialcomprises a mixture of 40%bis-(4''-n-octyloxybenzal-2-chlorophenylenediamine andp-methylbenzal-p''-n''butylaniline, these making up about 60 to 97percent of the total composition and p-cyanobenzal-p''-n-butylanilinecomprising the remaining 3 to 40 percent.
 3. The combination of claim 1wherein said polarizers comprise polarizing sheets on the sides of saidplates opposite said liquid crystal material.
 4. The combination ofclaim 1 wherein said polarizers comprise dichroic films.
 5. Thecombination of claim 4 wherein said dichroic films are deposited on saidconducting films and are in contact with the liquid crystal material. 6.The combination of claim 1 including means for continually varying saidelectric field whereby the colors viewed through said sandwich structurewill continually vary also.
 7. The combination of claim 1 wherein saidpolarizers are crossed at an angle of 45* with respect to said rubbedlines on the transparent conducting films.
 8. The combination of claim 1wherein said polarizers have their major axes both extending parallel tothe rubbed lines on said transparent conducting films.
 9. Thecombination of claim 1 wherein said electric field is varied in steps.